Figure 4.

Three surfaces (a) that fitted the ${\mathrm{s}}_{\mathrm{t}}^0{({\mathrm{W}}_{\text{AF},}\mathrm{w},10)}_{\text{MV}}$ data for the square field width $\mathrm{L}=4,10$, and 20 cm from the bottom to the top; three surfaces (b) that fitted the ${\mathrm{s}}_{\mathrm{t}}^0({\mathrm{W}}_{\text{AF},}\mathrm{w},\mathrm{L})$ data for square field sides 4, 10, and 20 cm from the bottom to the top.